It is well known in the petroleum field that petroleum which is found in subterranean reservoirs is recovered by many different methods. The primary method of petroleum recovery is by the primary recovery means which employs natural forces such as pressure, either by the petroleum itself or by the presence of gases, whereby petroleum is forced from the subterranean reservoir to the surface and recovered. Subsequent to the recovery of the petroleum by the primary means, due to the dissipation of the natural or gaseous pressure, more of the petroleum in the reservoir may be recovered by a secondary process in which water is forced into the reservoir to provide the pressure necessary to force the petroleum from the reservoir to the surface.
At some point in the recovery of petroleum, a state is reached in which it is more costly to use the water pumped in relative to the amount of oil which is recovered by this method. However, inasmuch as a relatively large amount of petroleum may still be present in the reservoir, either in a pool or by being trapped in interstices of relatively porous rock, it is necessary to effect the recovery of the petroleum by a tertiary method. The tertiary method or the enhanced oil recovery method may be effected by many different methods. For example, one tertiary recovery method may be thermal in nature in which steam is pumped into the reservoir to force the oil to the surface. However, some oil may be lost due to burning and, by combining the cost of the lost oil with the cost of the equipment and energy necessary to form the steam, may render such a method economically unattractive to operate. A second tertiary recovery method may comprise a fire flood method in which a portion of oil is ignited to create gases as well as reducing the viscosity of the heavy crude with a concomitant increase in pressure to force the oil from the reservoir. However, as in the method previously discussed, the drawback to this method is in the fact that some of the assets are being destroyed, thus increasing the cost of the operation. A third method for enhanced oil recovery is in the use of carbon dioxide to provide the pressure required to force the oil to the surface. In this method, the carbon dioxide is pumped into the oil reservoir to dissolve some of the heavies present which, in turn, will reduce the viscosity and allow the oil to reach the surface. The disadvantage which is present when utilizing such a method is the requirement for relatively expensive equipment to produce the carbon dioxide. In addition, the method is dependent upon the ready availability of carbon dioxide. Yet another method for enhanced oil recovery is found in the use of chemicals such as water-soluble polymers including polyacrylamide, biopolymers, etc. These polymers will increase the viscosity of the water in the solution and render the mobility ratio of water to oil whereby the solution will act more favorably as a plug.
Another method of effecting an enhanced oil recovery is by utilizing surfactants as a plug, whereby the oil which is present in the reservoir may be recovered by injecting an aqueous fluid containing a surfactant or a combination of surfactants along with other compounds into the reservoir. The use of surfactants in this system is necessary inasmuch as water alone does not displace petroleum with a relatively high degree of efficiency. This occurs due to the fact that water and oil are relatively immiscible and, in addition, the interfacial tension between water and oil is relatively high. The use of surfactants will lower or reduce the interfacial tension between the water and the oil, thus reducing the force which retains the oil which has been displaced in capillaries.
The prior art is replete with various surfactants which have been used in this tertiary system for the recovery of petroleum. One type of surfactant which has been employed in many processes involves a petroleum sulfonate. The sulfonated petroleum fractions have been obtained by sulfonating a crude oil. However, this crude oil feedstock contains a vast and varied number of chemical structures including aromatic hydrocarbons, paraffinic hydrocarbons, olefinic hydrocarbons, to name a few. However, a disadvantage in utilizing crude oil as a feedstock is that the feedstock usually does not contain a major portion of aromatic compounds which are the effective material which is sulfonated. As will hereinafter be shown, by utilizing certain linear alkylbenzene sulfonates which have been prepared from certain linear alkenes utilizing a specific type of catalyst, it is possible to obtain products which possess the desired physical characteristics necessary for lowering the interfacial tension between oil and water when used as one component of a surfactant slug.
As was previously discussed, prior U.S. patents teach the use of these petroleum sulfonates as one component of a surfactant mixture which may be used in a surfactant oil recovery process. For example, U.S. Pat. No. 4,214,999 discloses a surfactant fluid for use in flooding subterranean formations which contain petroleum comprising petroleum sulfonates possessing certain average equivalent weights and a solubilizing cosurfactant such as ethoxylated alkanols, sulfates or sulfonates. U.S. Pat. No. 4,013,569 also discloses a surfactant system for the recovery of petroleum utilizing a relatively water soluble aromatic ester polysulfonate as one component in the system. Another U.S. patent, namely U.S. Pat. No. 4,008,165, utilizes an aqueous surfactant containing fluid which includes a mixture of three surfactant materials including a sulfonate surfactant derived from an alkyl or alkylaromatic radical along with a phosphate ester surfactant and a sulfonated betaine, the system also being utilized in an oil recovery process.
Other U.S. patents disclose various water flooding methods for recovering oil such as in U.S. Pat. No. 3,874,454. This patent is concerned mainly with overbased formulations of sulfonates which are mixtures of alkali metal sulfonates plus a base component wherein the ratio: "weight of excess base component/weight of alkali metal sulfonate" has a value of about 0.03 to 2.0. These formulations are not merely neutralized but, as the term connotes, are overbased by an excess of the base component. The source of these sulfonates are petrochemical cuts of relatively ill-defined composition and may contain, among other products, such compounds as mono- or dialkylbenzenes as well as alkyl naphthalenes and alkylated tetrahydrated naphthalenes. U.S. Pat. No. 3,981,361 discloses an oil recovery method using a microemulsion containing a surfactant comprising synthetic sulfonates of o-xylene or toluene sulfonates. Likewise, U.S. Pat. No. 4,058,467 describes a method of oil recovery employing as a surfactant a carbon dioxide-saturated alkali metal hydrocarbon sulfonate water flooding additive. Again, the sulfonated products are overbased and, in addition, as hereinbefore set forth, are saturated with carbon dioxide.
As will hereinafter be shown in greater detail, it has now been found that by utilizing an aqueous surfactant slug in which one component thereof comprises sulfonates of a mixture of mono- and dialkyl aromatic hydrocarbons which have been obtained in an alkylation process utilizing a hydrogen fluoride catalyst, it is possible to effect a recovery of oil from a subterranean reservoir in a more efficient manner with a greater yield of tertiary oil products than has been obtained by utilizing surfactant slugs containing surfactants which are not the products of this type of alkylation reaction.
In one aspect an embodiment of this invention resides in a process for an enhanced oil recovery wherein an aqueous surfactant slug is introduced into a subterranean reservoir of oil to displace said oil from said reservoir, said slug being in a sufficient amount to lower the interfacial tension between said oil and water, the improvement which comprises utilizing as said surfactant slug an aqueous mixture comprising (a) from about 1% to about 10% of a sulfonate of a mixture of mono- and dialkyl-substituted aromatic hydrocarbons prepared by the alkylation of an aromatic hydrocarbon with a straight or branched chain olefinic hydrocarbon containing from about 6 to about 22 carbon atoms in the chain in the presence of a catalyst comprising hydrogen fluoride at alkylation conditions, (b) from about 1% to about 10% of a lower alkyl alcohol containing from about 3 to about 6 carbon atoms, and (c) from about 0.1% to about 2% of a nonionic ethoxylated normal alcohol containing from about 12 to about 15 carbon atoms.
A specific embodiment of this invention is found in a process for the enhanced oil recovery utilizing an aqueous surfactant slug for reducing the interfacial tension between oil and water, said slug comprising a mixture consisting of neutralizing sulfonates of a mixture of monoalkyl and dialkyl-substituted aromatic hydrocarbons which have been prepared by the alkylation of benzene with an olefinic hydrocarbon in the presence of hydrogen fluoride at a temperature in the range of from about ambient to about 100.degree. C. and a pressure in the range of from about atmospheric to about 50 atmospheres, said aromatic hydrocarbons being present in the reaction mixture in a molar ratio of about 3:1 to 1:10 moles of aromatic compound per mole of olefinic hydrocarbon, said sulfonate having been neutralized by the addition of a sodium compound, a lower alkyl alcohol containing from about 3 to about 6 carbon atoms, and a nonionic surfactant comprising an ethoxylated n-alcohol containing from about 12 to about 15 carbon atoms.
Other objects and embodiments will be found in the following further detailed description of the invention.